You use only to introduce the thing which must happen before the thing mentioned in the main part of the sentence can happen.
只有…才• The lawyer is paid only if he wins.
An only child is a child who has no brothers or sisters.
独生的• The actor, an only child, grew up in the Bronx. 那位演员是位独生子,在布朗克斯区长大。
Only can be used to add a comment which slightly changes or limits what you have just said.
(用来补充) 只是• It's just as dramatic as a film, only it's real. 这真和电影一样富有戏剧性,只不过它是现实。